(a) Technical Field
The present invention relates to a method for manufacturing a polylactic acid stereocomplex film using a solvent casting method.
(b) Background Art
Petrochemical-based plastics have long been used in our daily lives due to their advantages of mass producibility, lightness and low cost. They are disadvantageous in that they are not easily degraded and accumulate in the soil, unlike wood or other naturally-occurring materials. Also, due to a large amount of carbon dioxide emissions during incineration, such plastics have been blamed as one of the major culprits for global warming. Oil prices are expected to remain relatively high due to the depletion of fossil fuels available, and the regulations on the petrochemical-based plastics are becoming more strict to prevent global warming.
In this respect, resins or biodegradable plastics that can be degraded by microorganisms (bacteria, fungi, etc.) have been actively studied as post-petroleum materials. Examples of biodegradable resins include polyhydroxycarboxylic acid having aliphatic carboxyl ester units such as polylactic acid, polycaprolactone, and polyhydroxybutyrate.
Among them, polylactic acid has excellent heat resistance as well as well-balanced color and mechanical strength. Poly-L-lactic acid (hereinafter, also referred to as PLLA) or poly-D-lactic acid (hereinafter, also referred to as PDLA) has a melting point around 170° C. Despite the relatively high melting point, they have poor heat resistance when compared with the existing petrochemical-based polyesters represented by polyethylene terephthalate or polybutylene terephthalate, and thus are not suited for various applications. For instance, when polylactic acid is used to manufacture a food container, hot food cannot be put in the container. Accordingly, better heat resistance is required.
To improve the heat resistance, methods such as crosslinking, complexation (nanocomposites or stereocomplexes), high crystallization, etc., are employed. One of them is a method of preparing stereocomplex crystals having a melting point about 50° C. higher than that of homo-polylactic acid (PLA) by mixing the optical isomers poly-L-lactic acid and poly-D-lactic acid.
However, with this method, homo-PLA crystals are formed in addition to the stereocomplex crystals, and it is difficult to obtain a stereocomplex consisting only of stereocomplex crystals by heating. The presence of homo-PLA interferes with full utilization of high-melting properties of the stereocomplex. Therefore, a technique for manufacturing a PLA stereocomplex without containing homo-PLA is strongly needed.